Interferometric system for the detection and location of reflecting faults of light-guiding structures

ABSTRACT

An interferometric system for sensing and locating reflective defects in light-conducting structures comprises a mono-mode laser source (6), an incoherent source (4) with substantially the same central wavelength as the laser source, first and second couplers (10, 20) connected to the sources and to light sensors (52, 54), a first support (36) movable in one direction (D) and connected to the ends of the first and second couplers, a second support (38) oscillating in the same direction (D), reflectors (42, 43) attached to the second support opposite the ends of the first and second couplers. A third support support (40) is movable in the same direction (D) and connected to one end of the second coupler (20), a further stationary reflector (48) opposite said end, of the second coupler. The first coupler is connected to an optical waveguide (2), and devices (56, 58) for locating reflective defects in the waveguide.

BACKGROUND OF THE INVENTION

The present invention relates to an interferometric system for thedetection and location of reflecting faults in light-guiding structures.

The term "light-guiding structures" is understood to mean opticalwaveguides, such as e.g. optical fibers, optical couplers and evenlasers.

The present invention more particularly applies to the field of opticaltelecommunications and permits the location of weakly reflectingdiopters in such optical guides with a high resolution.

The invention also makes it possible to measure the transmissioncharacteristics of such optical guides, as well as the reflectioncoefficients of passive or active guiding structures.

An interferometric system for the detection and location of reflectingfaults is already known from the document "High-spatial-resolution andhigh-sensitivity interferometric optical-time-domain reflectometer",Masaru Kobayashi, Juichi Noda, Kazumasa Takada and Henry F. Taylor, SPIEConference, Orlando, Fla., Apr. 1-5, 1991, 1474-40.

SUMMARY OF THE INVENTION

The present invention solves the problem of obtaining an interferometricsystem able to accurately define the position of propagation "incidents"distributed along optical guides. For this purpose, the presentinvention makes use of a Michelson interferometer in incoherent light,as well as interferometric means with counting of interference fringesby laser.

More specifically, the present invention relates to an interferometricsystem for the detection and location of reflecting faults of lightguidance structures, the system being characterized in that it includes:

an incoherent light source,

a monomode laser source, whose wavelength is substantially equal to thecentral wavelength of the incoherent source,

first and second optical couplers, whose first respective branches areoptically coupled to the incoherent source and to the laser source,

a first support displaceable in translation in a given direction and towhich are fixed the ends of the second branches of the first and secondcouplers,

a second support able to oscillate in a given direction,

first and second light reflectors fixed to the second support andrespectively placed facing the ends of the second branches of the firstand second couplers in order to reflect there the light passing out ofthe same,

a third support displaceable in translation in the given direction andto which is fixed the end of a third branch of the second coupler,

a third light reflector fixed and positioned facing said end of thethird branch of the second coupler in order to reflect there the lightpassing out of the same, a third branch of the first coupler beingoptically coupled to the guiding structure,

first and second photodetectors respectively optically coupled to fourthbranches of the first and second couplers,

an interference fringe counter, whose input receives the signalssupplied by the second photodetector and

means for analyzing signals supplied by the first and secondphotodetectors, said analysis means serving to locate the reflectingfaults of the guidance structure, with the aid of appropriatedisplacements of the first, second and third supports and the counter.

The present invention also makes it possible, by using the signalprocessing by correlation, to lower the detection threshold or "minimumdetectable power" by at least one decade compared with theaforementioned known system and without having a prejudicial influenceon the spatial resolution.

The system according to the invention can also comprise a third opticalcoupler, whose first and second branches are respectively opticallycoupled to the incoherent source and to the laser source and whose thirdand fourth branches are respectively optically coupled to the firstbranches of the first and second couplers.

According to a special embodiment of the system according to theinvention, the analysis means incorporate a two-channel oscilloscoperespectively receiving the signal supplied by the first and secondphotodetectors, said oscilloscope displaying interferogramscorresponding to said signals.

The system according to the invention can also comprise piezoelectricmeans which are able to oscillate the second support in the givendirection.

According to a preferred embodiment of the system according to theinvention, the system includes means for regulating the speed of thedisplacement of the second support, the regulating means serving toimpose a constant displacement speed on the second support.

These regulating means can comprise a Michelson interferometer having alight source, whose coherence length is above the amplitude of thedisplacement of the second support, two arms respectively terminated bytwo light reflectors, whereof one is rendered rigidly integral with thesecond support and a third photodetector, as well as means forcontrolling the piezoelectric means as a function of the signal suppliedby said third photodetector, the control means imposing a constantdisplacement speed on the second support via the piezoelectric means.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein:

FIG. 1 A diagrammatic view of a special embodiment of the systemaccording to the invention.

FIG. 2 An interferogram obtained by means of the system according toFIG. 1 and which corresponds to a weakly reflecting diopter.

FIG. 3 Another interferogram obtained with a laser source used in saidsystem.

FIG. 4 A partial, diagrammatic view of means for regulating thedisplacement speed of the system plate shown in FIG. 1.

FIG. 5 Diagrammatically electronic means forming part of the regulatingmeans.

FIGS. 6A to 6C Diagrammatically the possibility of displacing aninterferogram on the screen of an oscilloscope of the system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An interferometric system according to the present invention, which isdiagrammatically shown in FIG. 1, is used for detecting and locating oneor more reflecting faults which an optical waveguide 2 my have. Thissystem also comprises an incoherent light source 4 constituted by anelectroluminescent diode emitter having a wide optical spectrum, as wellas a monomode laser emitter 6, whose wavelength is substantially equalto the central wave-length of the incoherent source 4.

The system of FIG. 1 also comprises first, second and third opticalcouplers 10, 20 and 30 of the 2×2 type and having monomode opticalfibers. Each optical fiber has four optical fiber branches which, forthe first coupler 10, carry the references 11, 12, 13 and 14, for thesecond coupler 20 the references 21, 22, 23 and 24 and for the thirdcoupler 30 the references 31, 32, 33 and 34.

The system according to FIG. 1 also comprises three supportsrespectively constituted by a first plate 36, which is displaceable intranslation parallel to a given direction D, a second plate 38 able tooscillate in the same direction D and a third plate 40 displaceable intranslation in the same direction D.

The first and third plates 36 and 40 are respectively associated withmotors 37 and 41 permitting their translation in the direction D and thesecond plate 38 is associated with piezoelectric means 39 able tooscillate the second plate 38 in said direction D.

The ends of the optical fibers 12 and 22 respectively belonging to thefirst and second couplers 10 and 20 are fixed to the plate 36 parallelto the direction D.

In the same way, one end of the optical fiber 23 belonging to the secondcoupler 20 is fixed to the third plate 40 parallel to the direction D.

On the second plate 38, which follows the first plate 36 and which facesthe first plate 36, are mounted light reflectors 42, 43, respectivelyfacing ends of the fibers 12 and 22. These light reflectors 42, 43reflect light beams passing out of the fibers 12, 22, so as to returnthe beams into the fibers 12, 22.

Optics 44, 45 are respectively fixed to the first and second plates 36,38, the optics 44 being on the first plate 36 positioned facing the endof the fiber 12 and the optics 45 being positioned on the second plate38 facing reflector 42, so that a light beam from the fiber 12 istransformed into a beam having parallel rays by the optics 44 and saidlatter beam is focussed on the reflector 42 by the optics 45, said beamthen returning into the optical fiber 12 by means of the optics 45, 44.

In the same way, optics 46, 47 are respectively fixed to the plates 36,38, the optics 46 being placed on the first plate 36 facing the end ofthe optical fiber 22 and the optics 47 being placed on the second plate38 facing the light reflector 43, the optics 46 transforming a lightbeam from the fiber 22 into a beam having parallel rays, which isfocussed onto the light reflector 43 by the optics 47, said beam beingreflected by the reflector 43 and returning into the optical fiber 22 bymeans of the optics 47 and then the optics 46.

There is a fixed light reflector 48 in front of the third plate 40. Thereflector 48 is more precisely positioned facing the end of the fiber23, which is fixed to the third plate 40. The reflector 48 reflects alight beam from the end of the fiber 23, so that said beam returns intothe said fiber.

An optics 49 is fixed to the third plate 40 facing the said end of thefiber 23 in order to transform a light beam emitted from the fiber 23into a beam having parallel rays.

An optics 50 is positioned facing the reflector 48 and is fixed withrespect to the reflector 48, so as to focus the beam with parallel raysonto the reflector 48, so that it returns into the end of the fiber 23via the optics 50 and then the optics 49.

The system shown in FIG. 1 also comprises two photodetectors 52, 54,which are respectively optically first and second coupled to the ends ofthe fibers 14, 24 belonging to the couplers 10, 20. Thus, thephotodetector 52 supplies an electric signal when it receives a lightbeam from the optical fiber 14 and the photodetector 54 supplies anelectric signal when it receives a light beam from the optical fiber 24.

Moreover, as can be seen in FIG. 1, the fibers 31, 32 are respectivelyoptically coupled to the incoherent source 4 and the laser source 6, theend of the optical fiber 13 of the first coupler 10 being opticallycoupled to on end of the optical guide 2 and the ends of the fibers 11and 21 of the first and second couplers 10 and 20 are respectivelyoptically coupled to the ends of the optical fibers 33, 34 of the thirdcoupler 30, so that the light beams are propagated in said ends of thefibers 33 and 34 and then respectively are able to pass into the fibers11 and 21 of the first and second couplers 10 and 20.

The system of FIG. 1 also comprises an interference fringe counter 56,whose input is connected to the output of the photodetector 54, as wellas a two-channel, digital oscilloscope 58, whereof one channel receivesthe output signal from the detector 52 via an amplifier 60 and whosesecond channel receives the output signal of the photodetector 54 via anamplifier 62.

The third coupler 30 supplies half the light intensity reaching it fromthe incoherent source 4 and/or the laser emitter 6 to the fiber 11 andthe other half of said intensity to the fiber 21.

The first optical coupler 10, which is an essential component of theinterferometric system diagrammatically shown in FIG. 1, serves as thebeam splitter of a Michelson interferometer. When the first coupler 10receives radiation by its fiber 11, it delivers half of this radiationto the fiber 12 and the other half to the fiber 13 and then mixes theradiation from the reflector 42 reaching it by the fiber 12 and theradiation reflected by one or more reflecting faults of the guide 2 andreaching it by the fiber 13, in order to supply the mixed radiation tothe photodetector 52.

The second optical coupler 20 also serves as a beam splitter for asecond Michelson interferometer. The second coupler 20 receivesradiation by its fiber 21 and delivers this radiation to the fiber 22and the other half to the fiber 23. The second coupler 20 mixes theradiation respectively reflected by the reflectors 43 and 48 andreaching it by the fibers 22 and 23 in order to deliver the thus mixedradiation to the photodetector 54.

Initially, the question as to whether a reflecting diopter is in theoptical waveguide 2 is ignored. If such a diopter exists, thephotodetector 52 will indicate its presence by supplying electricsignals leading to an interferogram of the type shown in FIG. 2 on theoscilloscope 58 (channel 1).

Such an interferogram is obtained by varying the distance between theend of the fiber 12, which is fixed to the first plate 36, and thereflector 42.

The interferogram of FIG. 2, which corresponds to a diopter having a lowreflection coefficient, is plotted in a marking system, whose ordinateaxis corresponds to the reflection amplitudes A and whose abscissa axiscorresponds to a distance covered by the second plate 38 and expressedin micrometers.

The maximum amplitude of the interferogram corresponds to the equalitybetween the optical length of the arm of the first Michelsoninterferometer, which is terminated by the light reflector 42, and theoptical length of the branch of the interferometer, which is terminatedby the reflecting diopter of the optical waveguide 2.

If the optical waveguide 2 has several reflecting diopters, obviouslyseveral successive interferograms will be obtained.

The form and amplitude of each interferogram is dependent on the opticalspectrum of the incoherent light source 4 used for producing theseinterferograms and is also dependent on the optical characteristics ofthe reflecting diopter, as well as the displacement speed of the lightreflector 42 with respect to the end of the optical fiber 12.

If a harmonic analysis in Fourier transform is made of the interferogramof FIG. 2, a component at frequency Fo (carrier frequency of theinterferograms) indicates the presence of a reflecting diopter, orreflecting center, with a signal quality which is better than in thecase of FIG. 2.

Thus, it is pointed out that all the interferograms "contain" a carrierfrequency close to Fo and it is sufficient to seek all the signalsspectrally centered on Fo in the photocurrent supplied by thephotodetector 52 in order to find all the reflecting diopters.

This frequency Fo is also the carrier frequency of the interferogramssupplied by the photodetector 54, because the latter interferograms areproduced during identical translations of the reflector 42 and thereflector 43.

Thus, the reflectors 42 and 43 are fixed to the second plate 38, whichis put into movement with the aid of the piezoelectric means 39 andtravel at the same speed and over the same distance.

The displacement of the first plate 36 is controlled by counting fringeswith the aid of the photodetector 54 in the second Michelsoninterferometer, when the laser emitter 6 is used as the light source.

FIG. 3 shows an interferogram obtained on the oscilloscope 58 (channel2) with the aid of the photodetector 54, when the laser emitter 6 isused as the light source and on the basis of this interferogram it ispossible to obtain information on the frequency Fo of the interferencefringes and which is the carrier frequency of the interferograms.

The collimating system formed by the end of the fiber 23 and the optics49 is kept fixed during the displacements of the first plate 36.

When the first plate 36 occupies the position in which the arms of thefirst interferometer, respectively terminated by the reflector 42 andthe reflecting diopter of the guide 2, have the same optical length (cf.hereinbefore), the third plate 40 is put into movement and thephotodetector 54 resumes its fringe counting in order to control thismovement until the third plate 40 reaches a position in which therespective optical lengths of the arms of the second interferometer,which are terminated by the light reflectors 43, 48, are equal. Whenthis position is reached, the laser emitter 6 is cut out and theelectroluminescent diode 4 is used as the light source for supplying alight beam into the fiber 21 of the second coupler 20.

The displacement of the plate 38 finally gives rise to twointerferograms, which are supplied by the photodetectors 52, 54 andwhose carrier frequencies are extremely close.

The first of these two interferograms is detectable under optimumconditions by correlation with the second interferogram and theappearance of this first interferogram is perfectly located by fringecoating in the second interferometer (having the second optical coupler20).

More precise details will be given hereinafter of the way of using theinterferometric system shown in FIG. 1.

The first step is to define an origin with respect to which the positionof the reflecting diopter of the optical waveguide 2 will be given. Thisorigin corresponds to the end of the optical waveguide 2, which isoptically coupled to the optical fiber 13 of the first coupler 10. Inorder to do this, the incoherent light source 4 is made to operate, thelaser emitter 6 being extinguished.

The first plate 36 is moved until the quality is obtained between theoptical lengths of the arms of the first interferometer, which arerespectively terminated by the reflector 42 and the reflecting dioptertaken as the origin (end of the guide 2). During these operations thesecond plate 38 is kept fixed.

On the oscilloscope 58 (channel 1) is obtained an interferogram, whosemaximum amplitude corresponds to equality between these optical lengths.

The first plate 36 is then immobilized in this position corresponding toequality of the optical lengths. The second plate 38 is oscillated. Themotor 37 of the first plate 36 is actuated so as to bring theinterferogram obtained to a chosen position on the screen of theoscilloscope 58. The first plate 36 is then stopped. The laser emitter 6is then made to function.

The third plate 40 is moved until equality of the optical lengths of thearms of the second interferometer is obtained and this is terminated bythe reflector 43 and the reflector 48.

This is detected by means of the photodetector 54 and an interferogram(channel 2) is obtained on the screen of the oscilloscope 58.

Coincidence is then brought about between the respective maxima of thetwo interferograms obtained on said screen, by displacing the thirdplate 40 over a length permitting this coincidence. The fringe counter56 is set to zero. The laser emitter 6 is extinguished. It is thenpossible to find the distance between the reflecting defect of theoptical waveguide 2 and the thus defined origin. To do this, the firstplate 36 is moved until detection takes place of the reflecting defector fault of the waveguide 2. This detection takes place when a newinterferogram appears on the channel 1 of the oscilloscope 58. The firstplate 36 is then stopped (with the second 38 still oscillating). Thelaser emitter 6 is put into operation.

The fringe counter 56 is actuated and the third plate 40 is displaceduntil coincidence is brought about between the maximum of theinterferogram of channel 2 and the maximum of the new interferogram ofchannel 1. The third plate 40 is then stopped .

However, each interference fringe is spaced from an adjacent fringe by alength equal to half the wavelength of the laser emitter 6.

Thus, it is possible to determine the distance between the origin andthe reflecting fault of the waveguide 2 by multiplying the number N ofinterference fringes counted by half said wavelength.

In order to make the oscillation speed of the third plate 38 as constantas possible, the third plate 38 is provided with means for regulatingthe speed. An optical method is advantageously used for measuring thedisplacement of the second plate 38.

Such a method makes it possible to measure displacements ofapproximately 0.1 micrometer in accordance with the wavelength of thelight source used for performing the optical method. The principle ofthe method is diagrammatically illustrated in FIG. 4.

This measurement is performed by using another Michelson interferometer82, which once again has a type 2×2 optical coupler 64 with opticalfibers, hereafter referred to as the fourth optical coupler.

The fourth coupler 64 has four branches or optical fibers, the firstbranch being coupled to a light source 66, the second to a receivingphotodiode 68 and the third to a total reflection mirror 70, which isfixed and which reflects into said branch the light passing out of thesame.

A mirror 72 is rigidly integral with the second plate 38, so that itmoves parallel to the direction D. The end of the fourth branch of thefourth coupler 64 is fixed and positioned facing the mirror 72.

Two fixed optics 74, 76 are placed between the mirror 72 and the end ofthe fourth branch of the fourth coupler 64.

The optics 76 transforms a beam from the end at the fourth branch into abeam having parallel rays. The optics 74 focusses the beam with parallelrays onto the mirror 72, which reflects it and returns it into the endof the fourth branch of the fourth coupler 64 via the optics 74 and 76.

The intensity I(t) of the light detected by the photodiode 68 andtherefore the intensity of the electric signal supplied by saidphotodiode 64 vary, as indicated hereinafter, as a function of the timet:

    I(t)=Io(1+((4πF(t)/1))

in which Io is a constant, π is 3.14 and l represents the wavelength ofthe light emitted from the light source 66.

The frequency F(t) is a function of the displacement speed of the secondplate 38 and the wavelength of the light source 66. In order to regulatethe speed, it is therefore sufficient to regulate the frequency of thesignal supplied by the photodiode 68.

It is necessary to use a light source 66, whose coherence length exceedsthe amplitude of the displacement of the second plate 38, thedisplacement being approximately 1 millimeter in the example described.

The light source 66 is, in exemplified manner, a laser diode DFB with awavelength of 1550 nanometers.

The frequency control device of the signal supplied by the photodiode 68is e.g. a phase locking control device, which is very simple toimplement. This device is diagrammatically illustrated by FIG. 5.

It comprises a phase comparator 78 receiving at its input a referencefrequency Fref, as well as the output signal of a Schnitt trigger, whoseinput is connected to the output of the photodiode 68.

In FIG. 5, block 82 represents the interferometer of FIG. 4.

The control device of FIG. 5 also comprises an integrator 84, whoseinput receives the output signal of the phase comparator 78, as well asan amplifier 86, which amplifies the output signal of the integrator 84.The output signal of the amplifier 86 controls the piezoelectric means39 associated with the second plate 38.

Without servocontrol, the variation of the speed obtained with thesepiezoelectric means 39 is approximately 25% (the photodiode 68 thensupplying a signal with a relatively wide spectrum).

However, the servocontrol described makes it possible to have afrequency variation smaller than 1 Hz around the reference frequency(e.g. 36 Hz, giving a speed of 28 micrometers per second), whichcorresponds to a speed variation below 2% (the spectrum of the signalsupplied by the photodiode 68 then being very narrow).

On returning to the system of FIG. 1, it will be shown hereinafter thatthe displacement of a plate effectively makes it possible to displace anintererogram on the oscilloscope 58, with reference to FIGS. 6A to 6C.The fiber 13 and the optical waveguide 2 remain fixed.

The arms of the first Michelson interferometer, respectively comprisingthe fibers 12 and 13, have optical lengths which are very close to oneanother and which, for each position of the first plate 36, areperfectly equal for a given position of the second plate 38.

An oscillogram is plotted during the displacement of the second plate 38(using the piezoelectric means 39) between two positions al and a2, forone position a of the first plate 36, while it is plotted during thedisplacement of the second plate 38 between two other positions b1 andb2, for another position b of the second plate 36.

FIG. 6A shows that the positions A and B corresponding to theinterferograms obtained for the positions a and b of the first plate 36are not positioned at the same locations on their respective segmentsala2, blb2.

According to FIG. 6A, if the oscillograms are initiated at al and bl,the interferograms corresponding respectively to the positions a and bof the first plate 36 are observed in the manner shown in FIGS. 6B and6C, where t represents the time and P the photocurrent obtained.

The following information is given in connection with the operation ofthe system of FIG. 1. The second plate 38 is driven, by piezoelectricmeans 39, in a continuous translation movement with a speed controllableby means of the reflector 43 (associated with the optics 47), whosemovement is controlled, in the interferometer having the optical coupler20, with the aid of the laser emitter 6, whose light reaches the secondcoupler 20 by the fiber 34, or with the aid of the laser emitter 4,whose light reaches the second coupler 20 by the optical coupler 30 andthe fiber 34.

Thus, the position of the second plate 38 can be perfectly known byusing the laser emitter 6 in the interferometer having the secondcoupler 20 and by counting the fringes passing during the displacementof the second plate 38, the third plate 40 being kept stationary.

As soon as the interferometer is balanced using the laser emitter 4coupled by means of the fiber 34 (the arms of the interferometercomprising the fibers 22 and 23 then having the same optical length),there is no further displacement in translation of either the thirdplate 40 or the first plate 36. The second plate 38 is displaced intranslation by the piezoelectric means 39. Thus, the photodetectors 52and 54 supply interferogram signals. The photodetector 54 supplies areference signal. The photodetector 52 supplies a signal characteristicof the optical reflection properties of the waveguide 2 and which iscompared with the reference signal by correlation, in order to obtain inaccurate manner the optical properties of the waveguide 2.

For example, if the waveguide 2 is a "perfect" mirror, as are thereflectors 42, 43 and 48 (respectively associated with the optics 45, 47and 50), the two interferograms are perfectly identical andsuperimposable.

It is on the basis of the differences between the signals that it ispossible to locate the diopter of the optical waveguide 2 and itsreflection coefficient is measured as a function of the wavelength.

We claim:
 1. Interferometric system for the detection and location ofreflecting faults of a light guiding structure (2), said systemcomprising:an incoherent light source (4), a monomode laser source (6),whose wavelength is substantially equal to a central wavelength of theincoherent source, first (10) and second (20) optical couplers, each ofsaid first and second couplers (10, 20) having first and secondbranches, said first branches (11, 21) being optically coupled to theincoherent light source and to the laser source, a first support (36)displaceable in translation in a given direction and to which are fixedends of the second branches (12, 22) of the first and second couplers, asecond support (38) adapted to oscillate in said given direction, first(42) and second (43) light reflectors fixed to the second support (38)and respectively placed facing the ends of the second branches (12, 22)of the first and second couplers in order to reflect the light passingout of the same, a third support (40) displaceable in translation in thegiven direction and to which is fixed an end of a third branch (23) ofthe second coupler (20), a third light reflector (48) fixed andpositioned facing said end of the third branch (23) of the secondcoupler (20) in order to reflect the light passing out of the end of thethird branch of the second coupler, a third branch (13) of the firstcoupler (10) being optically coupled to the guiding structure (2), first(52) and second (54) photodetectors respectively optically coupled tofourth branches (14, 24) of the first (10) and second (20) couplers, aninterference fringe counter (56) having an input which receives signalsfrom the second photodetector (54) and means (58) for analyzing signalssupplied by the first and second photodetectors (52, 54), said analyzingmeans serving to locate reflecting faults of the guiding structure (2),with the aid of appropriate displacements of the first, second and thirdsupports and the counter.
 2. An interferometric system according toclaim 1, further comprising a third optical coupler (30), whose first(31) and second (32) branches are respectively optically coupled to theincoherent source (4) and to the laser source (6) and whose third (33)and fourth (34) branches are respectively optically coupled to the firstbranches (11, 21) of the first (10) and second (20) couplers.
 3. Aninterferometric system according to claim 1, wherein the analysis meanscomprise a two-channel oscilloscope (58) respectively receiving thesignals supplied by the first (52) and second (54) photodetectors, saidoscilloscope displaying the interferograms corresponding to the signals.4. An interferometric system according to claim 1,k further comprisingpiezoelectric means (39) able to oscillate the second support (38) inthe given direction.
 5. An interferometric system according to claim 4,further comprising means (66, 68, 70, 72, 78, 80, 84, 86) for regulatingthe displacement speed of the second support (38), said regulating meansimposing a constant displacement speed on the second support.
 6. Aninterferometric system according to claim 5, wherein the regulatingmeans comprise a Michelson interferometer having a light source (66),whose coherence length is greater than an amplitude of displacement ofthe second support (38), two arms respectively terminated by two lightreflectors (70, 72), whereof one of said two light reflectors (72) isrendered rigidly integral with the second support (38) and a thirdphotodetector (68), as well as means (78, 80, 84, 86) for controllingthe piezoelectric means (39) as a function of the signals supplied bysaid third photodetector (68), said control means imposing a constantdisplacement speed on the second support via the piezoelectric means.